Conventionally, on a high-frequency circuit board with a high-frequency circuit on board, a bias line is provided on the substrate side for supplying power to the high-frequency circuit, etc. (Patent Document 1). As a high-frequency pulse radar for vehicle, for example, requires strongly small size substrates, a high-frequency circuit is mounted on one side of the substrate, while an antenna is mounted on the other side to downsize the substrate.
When both a high-frequency circuit and an antenna are mounted on one substrate as described above, it is necessary to prevent the bias line of the high-frequency circuit from being influenced by the electric wave from the antenna as much as possible. Therefore, the technology is known to reduce the influence of the electric wave from the antenna by embedding the bias line in the substrate. A multilayer substrate with three or more layers is used to embed the bias line in the substrate, and the bias line is provided in an internal layer of the multilayer substrate. By providing the bias line in the internal layer, the area needed for the substrate becomes smaller to achieve downsizing.
When the bias line is formed in the internal layer of the multilayer substrate, a microstrip line, a through hole, etc. are formed in the multilayer substrate as an transmission line for electrically connecting the bias line to the high-frequency circuit. The through hole is formed by forming a hole penetrating in the thickness direction of the substrate and coating its inner surface with a given metal. Such a penetrating through hole has the length almost as thick as the thickness of the substrate.